Methods of using aqueous gels

ABSTRACT

Carboxymethylhydroxyethyl cellulose in aqueous brine solutions is gelled by the addition of an alkaline earth metal hydroxide such as calcium hydroxide. The gelled compositions have utility as water diversion agents, pusher fluids, fracturing fluids, drilling muds, work-over fluids, and completion fluids.

This invention relates to new aqueous gels and uses thereof. Inaccordance with another aspect, this invention relates to gelled aqueousbrines containing carboxymethylhydroxyethyl cellulose (CMHEC) and agelling agent. In a further aspect, this invention relates to a methodof gelling brines comprising the addition of carboxymethylhydroxyethylcellulose (CMHEC) and a gelling agent. In accordance with a furtheraspect, this invention relates to the use of the instant compositions aswater diversion agents, pusher fluids, fracturing fluids, drilling muds,and work-over or completion fluids.

Accordingly, an object of this invention is to provide novel thickenedand gelled fluids.

A further object of this invention is to provide thickened and gelledfluids useful in the treatment of wells.

Other aspects, objects, and the several advantages of this inventionwill become apparent to those skilled in the art upon reading thisdisclosure and the appended claims.

In accordance with the invention, a gelled aqueous composition isprovided which comprises an aqueous brine solution containingcarboxymethylhydroxyethyl cellulose (CMHEC) and a sufficient amount ofalkaline earth metal hydroxide to cause gelation and form a gelledcomposition.

In accordance with one embodiment of the invention, brine solutions aregelled with carboxymethylhydroxyethyl cellulose (CMHEC) and an alkalineearth metal hydroxide, such as calcium hydroxide.

The aqueous gels of the invention are particularly useful in operationswherein a fluid medium is introduced into a bore hole in the earth as insecondary recovery operations, well treating operations, well completionoperations, etc.

Carboxymethylhydroxyethyl cellulose (CMHEC) polymers suitable for use inthe instant process are characterized by a degree of substitution (D.S.)in the broad range of about 0.1 to about 1, preferably about 0.2 toabout 0.5 and a molar substitution (M.S.) in the broad range of about 1to about 4, preferably about 2 to about 3. The "degree of substitution"designates the average number of hydroxyl groups per cellulosicanhydroglucose unit which are substituted with carboxymethyl groups. The"molar substitution" designates the average number of moles ofhydroxyethyl groups present per cellulosic anhydroglucose unit. Acommercially available polymer suitable for use in the invention isCMHEC of 0.4 carboxymethyl D.S. and 2.0 hydroxyethyl M.S.

The amount of cellulose ether (CMHEC) used in the practice of theinvention can vary widely depending upon the viscosity grade and purityof the ether, and properties desired in the aqueous gels of theinvention. In general, the amount of cellulose ether (CMHEC) used willbe a water thickening amount, i.e., at least an amount which willsignificantly thicken the water to which it is added. Generallyspeaking, amounts in the range of about 0.5 to about 4 lbs/barrel,preferably from about 2 to 3 lbs/barrel of solution will be used.However, amounts outside these ranges can be used.

The term "increased viscosity" or the term "thickened" as used herein inthe descriptions with respect to the aqueous solutions, is intended tomean any increase in the viscosity of a solution of the polymer even upto the point where it is no longer flowable. Thus, solutions can beprepared covering a wide range of viscosities depending on theapplication intended.

As used herein, the term "brine" is intended to include any aqueoussolution of mineral salts having greater than 1,000 ppm solids content,such as are frequently present in oil fields. Oil field brines commonlycontain varying amounts of sodium chloride, calcium chloride, ormagnesium salts. Also, flooding solutions are frequently modified by theaddition of potassium chloride to stabilize the subsurface clay.Accordingly, potassium chloride is also frequently encountered. Inpractical applications, such as the post-primary petroleum recoveryoperations, the naturally occurring brine which is encountered will bepredominantly sodium chloride or potassium chloride but a great numberof other salts are almost always present.

The aqueous medium in the present invention is preferably a hard brinecomprising on the order of 1,000 to 16,000 ppm divalent cations such aspreferably Ca++ with a total dissolved solids (TDS) level in the rangeof 100,000 to 300,000 ppm. It is contemplated that the inventivecompositions would be most applicable in areas characterized by theavailability of highly saline field waters.

The gelling agents, according to the invention, are the alkaline earthmetal (Group IIA) hydroxides, preferably calcium hydroxide. The amountof gelling agent used in the practice of the invention will be a smallbut finite amount which is effective or sufficient to cause gelationwhen the alkaline earth metal hydroxide is mixed with the remainingingredients of the aqueous composition. In general, the amount ofgelling agent added will range from about 0.25 to about 4 lbs/barrel,preferably about 0.5 to about 3 lbs/barrel of solution.

Various methods can be used for preparing the aqueous gels of theinvention. The alkaline earth metal hydroxide can be first added to asolution of CMHEC in the brine. Generally speaking, where convenient,the preferred method is to first disperse the CMHEC in the brine andthen add the alkaline earth metal hydroxide to gel the solution.

As water diversion agents and pusher fluids, respectively, the inventivecompositions can be injected at injection wells to correct thepermeability profile of a formation or for mobility control in apost-primary oil recovery operation. The inventive compositions canfunction as permeability correction agents by temporarily decreasingwater permeability in "thief zones" (high permeability areas) and thusdirect the subsequently injected drive fluids to the less permeable,relatively oil-rich zones. The effectiveness of such agents is reflectedby the magnitude of the increase observed in the oil-to-water ratio atsurrounding production wells. The instant compositions can function aspusher fluids (mobility buffers), e.g., in surfactant flooding byinjecting the compositions as viscous slugs to push the previouslyinjected surfactant slug and recovered oil to a producing well. Theviscosity of the pusher fluid relative to that of the surfactant slug,is such that little or no fingering of the slug into the surfactant slugtakes place.

As hydraulic fracturing fluids, the inventive compositions can beemployed to increase the production of oil from subterranean formations.Hydraulic fracturing comprises the injection of a suitable fracturingfluid down a well penetrating a formation and into said formation undersufficient pressure to fracture the formation. The resulting crack orfracture in the formation provides a passageway which facilitates flowof fluids through the formation and into the well. Propping agents canbe included in the inventive compositions if desired. Propping agentswhich can be used include any of those known in the art, e.g., sandgrains, walnut shell fragments, tempered glass beads, aluminum pellets,and similar materials, so long as they meet the appropriatecompatibility requirements. Generally speaking, it is desirable to usepropping agents having particle sizes in the range of 8 to 40 mesh (U.S.Sieve Series). However, particle sizes outside this range can beemployed. The observed gel strengths of the novel composition (see TableI) indicate its superior proppant-carrying ability.

It is contemplated that various known agents can be added to theinventive compositions to minimize fluid loss to the formation duringfracturing operations. Apparently, these fluid loss additives decreasethe transfer of water into the formation by temporarily plugging theexposed faces of the more accessible channels and passages in theformation. One of the ways this is done is by increasing the viscosityof the fracturing fluid. The viscosity of the inventive fluid aids inminimizing this fluid loss.

It is contemplated that the instant compositions have application in thearea of drilling muds, work-over fluids and completion fluids. Thesuspending capacity of the fluids can be enhanced by the presence ofcalcium ions under alkaline conditions which may be effected by theaddition of lime to the fluids containing CMHEC. A mud with goodsuspending capacity can convey the large cuttings to the surface wheremost of the cuttings can be removed. The mud containing the cuttingsmust stand in a settling pit to allow the cuttings sufficient time tosettle out, or the cuttings can be separated by cyclones orcentrifuging.

The thickened compositions of the invention intended for use asfracturing, water diversion or mobility buffer fluids can be prepared onthe surface in a suitable tank equipped with suitable mixing means, andthen pumped down the well and into the formation employing conventionalequipment for such compositions. However, it is within the scope of theinvention to prepare the compositions while they are being pumped downthe well. This technique is sometimes referred to as "on the fly." Forexample, an aqueous saline solution of the polymer can be prepared in atank adjacent the well head. Pumping of this solution through a conduitto the well head can then be started. Then, a few feet downstream fromthe tank, a suitable connection can be provided for introducing anaqueous solution of an alkaline earth metal hydroxide such as calciumhydroxide. As will be understood by those skilled in the art, the rateof introduction of the components into the conduit will depend upon thepumping rate of the inventive composition through the conduit. Mixingorifices can be provided in the conduit, if desired.

When prepared for use in drilling, completion and work over fluids, thematerials of this invention, can be prepared using conventional mudmixing equipment. Polymer is generally added to the drilling mud throughjet hoppers discharging into the suction pit. Lime can also be mixedthrough this jet hopper. The materials are in turn mixed with thedrilling mud by either jet or paddle mixers. Hydration occurs as the mudis being mixed and pumped through the bit nozzles.

It is within the scope of the invention to precede the injection of thethickened composition as a fracturing fluid into the well and out intothe formation with a preflush of a suitable cooling fluid, e.g., water.Such fluids serve to cool the well tubing and formation and extend theuseful operating temperature range of the compositions. The volume ofthe cooling fluid so injected can be any suitable volume sufficient tosignificantly decrease the temperature of the formation being treated,and can vary depending upon the characteristics of the formation. Forexample, amounts up to 20,000 gallons, or more, can be used to obtain atemperature decrease on the order of 100° to 250° F.

The following example further illustrates but should not be consideredas unduly limiting on the invention.

EXAMPLE I

A hard brine solution was prepared by mixing 3150 mL of saturated sodiumchloride solution (26 weight percent NaCl), 350 mL deionized water and150 g calcium chloride. Various amounts of carboxymethylhydroxyethylcellulose (CMHEC) were added to samples of the hard brine and thesemixtures were gelled by the addition of hydrated lime. Apparentviscosities and gel strengths of the thickened fluids were determinedand the results are tabulated in Table I.

                                      TABLE I                                     __________________________________________________________________________    Apparent Viscosities and Gel Strengths of                                     CMHEC/Hard Brine Mixtures Gelled With Lime                                             Concentration                                 Gel Strengths,         Run      (lb/bbl).sup.a                                                                         Apparent Viscosities (cp)            lbs/100 ft..sup.2      No.                                                                              Run Type                                                                            CMHEC.sup.b                                                                         Lime                                                                             51 Sec.sup.-1                                                                      102 Sec.sup.-1                                                                      170 Sec.sup.-1                                                                      340 Sec.sup.-1                                                                      511 Sec.sup.-1                                                                       1022 Sec.sup.-1                                                                      (10 sec/10             __________________________________________________________________________                                                           min.)                  1  Control                                                                             3     0   540  350  252   160.5 123    80.5   19/17                  2  Invention                                                                           3     2  1550 1050  726   513   352    181.5  45/92                  3  Control                                                                             4     0  1050  640  444   273   208    126.5  45/45                  4  Invention                                                                           4     0.5                                                                              1660 1100  750   465   332    180    65/82                  __________________________________________________________________________     .sup.a 1 g reagent per 350 mL solution is equivalent to 1 lb. reagent per     barrel of solution, e.g., 150 g CaCl.sub.2 in 3150 mL saturated NaCl          solution + 350 ml deionized water is equivalent to 15 lbs. CaCl.sub.2 per     barrel of solution.                                                           .sup.b CMHEC420 was used in these runs. CMHEC420 is                           carboxymethylhydroxyethyl cellulose with a 0.4 degree of substitution and     a 2 value for molar substitution.                                        

Referring to Table I, the greater gel strengths of the inventive runs(No. 2 and No. 4) compared, respectively, to the control runs (No. 1 andNo. 3) indicate that the inventive thickened fluids could advantageouslybe used to suspend cuttings in drilling muds and proppants in fracturingfluids. The apparent viscosities show that all the inventive and controlfluids are thixotropic (shear-thinning). A comparison of the apparentviscosities exhibited by the inventive runs (No. 2 and No. 4)respectively, with the control runs (No. 1 and No. 3) indicate that thelime treatment greatly enhanced the apparent viscosity of the inventionruns. This suggests in particular that the inventive compositions wouldbe suitable for use as mobility buffers in carbonate reservoirs.

I claim:
 1. In a method wherein a fluid medium is introduced into aborehole in the earth and into contact with the pores of a subterraneanformation penetrated by said borehole, the improvement wherein at leasta portion of said fluid medium comprises a gelled aqueous brine solutionhaving at least 1,000 ppm divalent cations containing an effective waterthickening amount of CMHEC characterized by a degree of substitution(DS) ranging from about 0.1 to about 1 and a molar substitution (MS)ranging from about 1 to about 4 and an effective gelling amount of analkaline earth metal hydroxide.
 2. A method according to claim 1whereinsaid borehole comprises an injection well, said formationcomprises an oil-bearing formation and is also penetrated by aproduction well, said method is a fluid drive process carried out forthe recovery of oil from said formation by injecting a drive fluid intosaid formation via said injection well and driving oil to saidproduction well, and said fluid medium comprising said aqueous gel isintroduced into said injection well and into the pores of saidformation.
 3. A method according to claim 2 wherein said fluid drivefunctions as a mobility buffer and follows injection of a surfactantslug.
 4. A method according to claim 1 wherein said method comprises amethod for drilling a borehole into the eartha drill bit is actuated insaid borehole in said contact with said formation and penetrates same inthe presence of said fluid medium, and said fluid medium, together withformation solids entrained therein, is removed from said borehole.
 5. Amethod of fracturing in a well traversing a formation which comprisesinjecting into the well a fracturing fluid wherein at least a portion ofsaid fracturing fluid passing through the pores of said formationcomprises a gelled aqueous brine solution having at least 1,000 ppmdivalent cations containing an effective water thickening amount ofCMHEC characterized by a degree of substitution (DS) ranging from about0.1 to about 1 and a molar substitution (MS) ranging from about 1 toabout 4 and an effective gelling amount of an alkaline earth metalhydroxide.
 6. A method according to claim 5 wherein the fluid contains agranular material (propping agent).
 7. The method of treating a welltraversing a producing formation which comprises injecting a fracturingfluid into the pores of said formation wherein at least a portionthereof comprises a gelled aqueous brine solution having at least 1,000ppm divalent cations containing an effective water thickening amount ofCMHEC characterized by a degree of substitution (DS) ranging from about0.1 to about 1 and a molar substitution (MS) ranging from about 1 toabout 4 and an effective gelling amount of an alkaline earth metalhydroxide into a confined zone of said well at a rate sufficient toincrease the pressure exerted in said zone until formation fractureoccurs, continuing to inject said fluid into the formation, thereafterpermitting the gel to break and producing the well.
 8. A methodaccording to claim 7 wherein the fluid contains a granular material(propping agent).